In metal processing, various thermal treatment operations such as annealing, hardening, brazing and sintering are often performed under inert or reducing atmospheres in order to avoid and/or remove oxidation from metal parts. In powder metallurgy, for example, high pressure is applied to metal powders to form compacts which are then sintered in a furnace to form metal parts. Sintering of the compacts is typically performed under an inert or reducing atmosphere such as a mixture of nitrogen (N2) and hydrogen (H2) or an Endo gas mixture containing carbon monoxide (CO), H2 and N2. The sintered metal parts are then subjected to a cooling phase, during which transformation of the microstructure of the metal parts may occur. Certain metal parts may also be subjected to sinterhardening, i.e., transformation to a hard martensite phase during cooling. Sinterhardening is typically carried out in a cooler such as a convection cooler, with alloy additives such as nickel, molybdenum, among others, added to the metal powders prior to sintering. These alloy additives are used to facilitate sinterhardening of the metal parts, resulting in products that are either harder or tougher than non-sinterhardened parts. Water coolers, which provide slower cooling than convection coolers, may also be used with more expensive types of powder mix to provide metal parts with increased martensite phase.
Much of the efforts for improving sintering methods have focused on the control of process conditions during sintering. However, since the transformation of microstructure during the cooling phase directly affects the material properties of the processed parts, there is a need for an improved method of sintering by controlling process conditions during the cooling phase.